One goal of loudspeaker systems is to achieve a low resonant frequency in a speaker enclosure that has a relatively small internal volume. The resonant frequency of a speaker enclosure can be decreased by adding an air adsorber to the enclosure; the adsorber acts to increase air compliance of the enclosure.
A good adsorption material should have at least the following characteristics. The amount of gas adsorbed and desorbed should be strongly dependent on the pressure change. Also, the adsorption properties of the material should not degrade or change significantly when exposed to different environmental conditions.
Moisture in the atmosphere is one of the key elements detrimental to the proper function of adsorption materials. Often when an adsorption material is exposed to a humid environment many adsorption sites are preferentially occupied by water molecules, leaving few sites for the adsorption/desorption of air molecules. This phenomenon renders the adsorption material ineffective in regulating the pressure of an acoustic enclosure through adsorption/desorption of air inside the enclosure.
Zeolites are silicates that tend to adsorb water vapor from the environment; some zeolites are even used as drying agents. Many zeolites will also adsorb and desorb air from the atmosphere. The adsorption behavior of zeolite depends on both the structure and composition. Aluminum is commonly present in silicate zeolites. Since aluminum has a different oxidation state than silicon (Al is +3 and Si is +4), its presence creates local polar sites. And, since water vapor is polar, zeolites with aluminum tend to preferentially adsorb water vapor over air. Water vapor adsorption decreases the ability of zeolites to adsorb air and accordingly decreases their ability to enhance the acoustic compliance of loudspeaker enclosures.
Highly pure silicate zeolites with a Si/Al mole ratio of over 400 have been proposed for use as air adsorbents in loudspeaker enclosures. Such highly pure zeolites are relatively expensive to manufacture because they require high-purity silica source material; this makes their adoption as adsorbers in loudspeaker enclosures less desirable.